Composite material, and manufacturing method and uses of same

ABSTRACT

Provided are a composite material excellent in plastic workability, a method of producing the composite material, a heat-radiating board of a semiconductor equipment, and a semiconductor equipment to which this heat-radiating board is applied. 
     This composite material comprises a metal and an inorganic compound formed to have a dendritic shape or a bar shape. In particular, this composite material is a copper composite material, which comprises 10 to 55 vol. % cuprous oxide (Cu 2 O) and the balance of copper (Cu) and incidental impurities and has a coefficient of thermal expansion in a temperature range from a room temperature to 300° C. of from 5×10 −6  to 17×10 −6 /° C. and a thermal conductivity of 100 to 380 W/m·k. This composite material can be produced by a process comprising the steps of melting, casting and working and is applied to a heat-radiating board of a semiconductor article.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a new composite material and, moreparticularly, to a copper composite material of low thermal expansionand high thermal conductivity, and various kinds of uses such as asemiconductor equipment in which this composite material is used.

2. Description of the Prior Art

Techniques related to the conversion and control of electric power andenergy by means of electronic devices and, in particular, powerelectronic devices used in an on-off mode and power conversion systemsas applied techniques of these power electronic devices are called powerelectronics.

Power semiconductor devices with various kinds of on-off functions areused for power conversion. As such semiconductor devices, there are putto practical use not only rectifier diodes which contain pn junctionsand which have conductivity only in one direction, but also thyristors,bipolar transistors, MOS FETs (metal oxide semiconductor field effecttransistors) and etc. which differ from each other in variouscombinations of pn junctions. Moreover, there are also developedinsulated gate type bipolar transistors (IGBTS) and gate turn-offthyristors (GTOs) which have a turn-off function by gate signals.

These power semiconductor devices causes the generation of heat byenergization and the amount of generated heat tends to increase becauseof the high capacity design and high speed design of power semiconductordevices. In order to prevent the deterioration of the properties of asemiconductor device and the shortening of its service life from beingcaused by the heat generation, it is necessary to provide aheat-radiating portion to thereby suppress a temperature rise in andnear the semiconductor device. Because copper has a high thermalconductivity of 393 W/m·k and is inexpensive, this metal is generallyused as heat-radiating members. However, because a heat-radiating memberof a semiconductor equipment provided with a power semiconductor deviceis bonded to Si having a thermal expansion coefficient of 4.2×10⁻⁶/° C.,a heat-radiating member having a thermal expansion coefficient close tothis value is desired. Because the thermal expansion coefficient ofcopper is as large as 17×10⁻⁶° C., the solderability of copper to thesemiconductor device is not good. Therefore, materials with acoefficient of thermal expansion close to that of Si, such as Mo and W,are used as a heat-radiating member or installed between thesemiconductor device and the heat-radiating member.

On the other hand, integrated circuits (ICs) formed by integratingelectronic circuits on one semiconductor chip are sorted according totheir functions into a memory, logic, microprocessor, etc. They arecalled electronic semiconductor devices in contrast with powersemiconductor devices. The integration degree and operating speed ofthese semiconductor devices have been increasing year by year, and theamount of generated heat has also been increasing accordingly. On theother hand, an electronic semiconductor device is generally housed in apackage in order to prevent troubles and deterioration by shutting itoff from the surrounding atmosphere. Most of such packages are either aceramic package or a plastic package, in which ceramic package asemiconductor device is die-bonded to a ceramic substrate and sealed,and in which plastic package a semiconductor device is encapsulated withresins. In order to meet requirements for higher reliability and higherspeeds, a multi-chip module (MCM) in which multiple semiconductordevices are mounted on one substrate is also manufactured.

In a plastic package, a lead frame and terminals of a semiconductordevice are connected by means of a bonding wire and encapsulated withplastics. In recent years, with an increase in the amount of generatedheat of semiconductor devices, a package in which the lead frame has aheat-dissipating property and another package in which a heat-radiatingboard for heat dissipation is mounted have also come to be thought of.Although copper-base lead frames and heat-radiating boards of largethermal conductivity are frequently used for heat dissipation, there issuch a fear as problems may occur due to a difference in thermalexpansion from Si.

On the other hand, in a ceramic package, a semiconductor device ismounted on a ceramic substrate on which wiring portions are printed, andthe semiconductor device is sealed with a metal or ceramic cap.Moreover, a composite material of Cu—Mo or Cu—W or a kovar alloy isbonded to the ceramic substrate and used as a heat-radiating board, andin each of these materials there is required an improvement inworkability and a low cost as well as lower thermal expansion design andhigher thermal conductivity design.

In an MCM (multi-chip module), multiple semiconductor devices aremounted as bare chips on the thin-film wiring formed on an Si or a metalor a ceramic substrate, are housed in a ceramic package, and areencapsulated with a lid. When the heat-radiating property is required, aheat-radiating board and a heat-radiating fin are installed in thepackage. Copper and aluminum are used as the material for metalsubstrates. Although copper and aluminum have the advantage of a highthermal conductivity, these metals have a large coefficient of thermalexpansion and have inferior compatibility with semiconductor devices.For this reason, Si and aluminum nitride (AlN) are used as the substrateof a high-reliability MCM. Further, because the heat-radiating board isbonded to the ceramic package, a material having good compatibility withthe package material in terms of coefficient of thermal expansion andhaving a large thermal conductivity is desired.

As mentioned above, all semiconductor equipments each provided with asemiconductor device generate heat during operation, and the function ofthe semiconductor device may be impaired if the heat is accumulated. Forthis reason, a heat-radiating board with excellent thermal conductivityfor dissipating the heat to the outside is necessary. Because aheat-radiating board is bonded directly or via an insulating layer tothe semiconductor device, its compatibility with the semiconductordevice is required not only in thermal conductivity, but also in thermalexpansion.

The materials for semiconductor devices presently in use are mainly Si(silicon) and GaAs (gallium arsenide). The coefficients of thermalexpansion of these two materials are 2.6×10⁻⁶/° C. to 3.6×10⁻⁶/° C. and5.7×10⁻⁶/° C. to 6.9×10⁻⁶/° C., respectively. As the materials forheat-radiating boards having a coefficient of thermal expansion close tothese values, AlN, SiC, Mo, W, Cu—W, etc., have been known. However,because each of them is a single material, it is difficult to control toan arbitrary level the coefficients of heat transfer and thermalconductivity and, at the same time, there is a problem that they arepoor in workability and require a high cost.

Recently, Al—SiC has been proposed as a material for heat-radiatingboards. This is a composite material of Al and SiC and the coefficientsof heat transfer and thermal conductivity can be controlled in a widerange by changing the proportions of the two components. However, thismaterial has the disadvantage of very inferior workability and a highcost. A Cu-Mo sintered alloy is proposed in JP-A-8-78578, a Cu—W—Nisintered alloy being proposed in JP-A-9-181220, a Cu—SiC sintered alloybeing proposed in JP-A-9-209058, and-an Al—SiC is proposed inJP-A-9-15773. In these publicly known composite materials obtained bypowder-metallurgical processes, the coefficient of thermal expansion andthermal conductivity can be controlled in wide ranges by changing theratio of the two components. However, their strength and plasticworkability are low and the manufacture of sheets is difficult. Inaddition, there are problems of a high cost related to the production ofpowder, an increase in the steps of manufacturing process and etc.

SUMMARY OF THE INVENTION

The object of the invention is to provide a composite material excellentin plastic workability, a method of manufacturing the compositematerial, a semiconductor equipment in which the composite material isused, a heat-radiating board of the semiconductor equipment, anelectrostatic adsorption device, and a dielectric board of theelectrostatic adsorption device.

As a result of a repetition of various researches, the present inventorshave found that the above problems can be solved by a composite materialcomposited through the steps of melting Cu of high thermal conductivityand Cu₂O of lower thermal expansion than Cu and dispersing each of thesematerials.

According to the first aspect of the invention, there is provided acomposite material comprising a metal and an inorganic compoundpreferably having a smaller coefficient of thermal expansion than themetal, most of the compound being granular grains with a grain size ofpreferably not more than 50 μm and dendrites.

According to the second aspect of the invention, the compound comprisesdendrites each having a bar-like stem and branches of a granular shape.

According to the third aspect of the invention, there is provided acomposite material comprising a metal and an inorganic compound, most ofthe compound are granular grains with a grain size of 5 to 50 μm anddendrites, and 1 to 10% of the whole compound are fine grains with agrain size of not more than 1 μm.

According to the fourth aspect of the invention, there is provided acomposite material comprising a metal and an inorganic compound, thecoefficient of thermal expansion or thermal conductivity being larger ina solidification direction than in a direction vertical to thesolidification direction.

Most preferably, the composite material of the invention may be onecomprising copper and copper oxide.

According to the fifth aspect of the invention, there is provided acomposite material comprising a metal and an inorganic compound having ashape of bar with a diameter of 5 to 30 μm, and preferably, not lessthan 90% of the whole of the inorganic compound in terms of the areapercentage of section is in the shape of a bar with a diameter of 5 to30 μm.

The composite material of the invention may comprise copper and copperoxide and may be plastically worked.

According to the sixth aspect of the invention, there is provided acomposite material comprising copper, copper oxide and incidentalimpurities, the content of the copper oxide being 10 to 55 % by volume,copper oxide being made to be dendrites, the coefficient of linearexpansion in a temperature range from room temperature to 300° C. being5×10⁻⁶/° C. to 17×10⁻⁶/° C., and the thermal conductivity thereof atroom temperature is 100 to 380 W/m·k. This composite material hasanisotropy.

According to the seventh aspect of the invention, there is provided acomposite material comprising copper, copper oxide, preferably cuprousoxide (Cu₂O) and incidental impurities, the content of copper oxidebeing preferably 10 to 55 % by volume, the copper oxide being providedwith a shape of bars each oriented in one direction, the coefficient oflinear expansion of the copper oxide in a temperature range from roomtemperature to 300° C. is 5×10⁻⁶/° C. to 17×10⁻⁶/° C., and the thermalconductivity thereof at room temperature is 100 to 380 W/m·k. In thiscomposite material, the thermal conductivity in the oriented directionis higher than that in the direction at right angles to the orienteddirection, and the difference between the two is preferably 5 to 100W/m·k.

According to the eighth aspect of the invention, there are provided amanufacturing method in which a metal and an inorganic compound forminga eutectic structure with this metal are melted and solidified and, inparticular, a manufacturing method of a composite material comprisingcopper and copper oxide. This manufacturing method preferably comprisesthe step of preparing a raw material of copper or copper and copperoxide, melting the raw material in an atmosphere having a partialpressure of oxygen of 10⁻² Pa to 10³ Pa followed by casting, performingheat treatment thereof at 800° C. to 1050° C., and preferably performingcold or hot plastic working thereof.

According to the ninth aspect of the invention, there is provided aheat-radiating board for semiconductor equipment, which board is made ofthe above composite material. In the heat-radiating board for thesemiconductor equipment may have a nickel plating layer on its surface.

According to the tenth aspect of the invention, there is provided asemiconductor equipment comprising an insulating substrate mounted on aheat-radiating board, and a semiconductor device mounted on theinsulating substrate, said heat-radiating board being the same asrecited in the ninth aspect of the invention.

According to the eleventh aspect of the invention, there is provided asemiconductor equipment comprising a semiconductor device mounted on aheat-radiating board, a lead frame bonded to the heat-radiating board,and a metal wire for electrically connecting the lead frame to thesemiconductor device, the semiconductor device being resin-encapsulated,and the heat-radiating board being the same as recited in the ninthaspect of the invention.

According to the twelfth aspect of the invention, there is provided asemiconductor equipment which comprises a semiconductor device mountedon a heat-radiating board, a lead frame bonded to the heat-radiatingboard, and a metal wire for electrically connecting the lead frame tothe semiconductor device, the semiconductor device beingresin-encapsulated, at least the face of the heat-radiating board whichface is opposed to the connection face of the semiconductor device isopened, and the heat-radiating board being the same as recited in theninth aspect of the invention.

According to the thirteenth aspect of the invention, there is provided asemiconductor equipment comprising a semiconductor device mounted on aheat-radiating board, a ceramic multilayer substrate provided with a pinfor connecting external wiring and an open space for housing thesemiconductor device in the middle thereof, and a metal wire forelectrically connecting the semiconductor device to a terminal of thesubstrate, and both of the heat-radiating board and the substrate beingbonded to each other so that the semiconductor device is installed inthe open space, the substrate being bonded to a lid so that thesemiconductor device is isolated from an ambient atmosphere, and theheat-radiating board being the same as recited in the ninth aspect ofthe invention.

According to the fourteenth aspect of the invention, there is provided asemiconductor equipment comprising a semiconductor device mounted on aheat-radiating board, a ceramic multilayer substrate having a terminalfor connecting external wiring and a concave portion for housing thesemiconductor device in the middle of the substrate, and a metal wirefor electrically connecting the semiconductor device to the terminal ofthe substrate, both of the heat-radiating board and the substrate beingbonded to each other so that the semiconductor device is installed inthe concave portion of the substrate, the substrate being bonded to alid so that the semiconductor device is isolated from an ambientatmosphere, and the heat-radiating board is the same as recited in theninth aspect of the invention.

According to the fifteen aspect of the invention, there is provided asemiconductor equipment comprising a semiconductor device bonded to aheat-radiating board with a heat-conducting resin, a lead frame bondedto a ceramic insulating substrate, a TAB for electrically connecting thesemiconductor device to the lead frame, both of the heat-radiating boardand the insulating substrate being bonded to each other so that thesemiconductor device is isolated from an ambient atmosphere, and anelastic body of heat-conducting resin interposed between thesemiconductor device and the insulating substrate, the heat-radiatingboard being the same as recited in the ninth aspect of the invention.

According to the sixteenth aspect of the invention, there is provided asemiconductor equipment comprising a semiconductor device bonded onto afirst heat-radiating board by use of a metal, a second heat-radiatingboard to which an earthing board is bonded, the first heat-radiatingboard being mounted on the earthing board, and a TAB electricallyconnected to a terminal of the semiconductor device, the semiconductordevice being encapsulated by resin, the heat-radiating board being thesame as recited in the ninth aspect of the invention.

According to the seventeenth aspect of the invention, there is provideda dielectric board for electrostatic adsorption devices which board ismade of the composite material recited above.

According to the eighteenth aspect of the invention, there is providedan electrostatic adsorption device in which, by applying a voltage to anelectrode layer, an electrostatic attractive force is generated betweena dielectric board bonded to the electrode layer and a body to therebyfix the body to the surface of the dielectric board, the dielectricboard being the same as the dielectric board recited in the seventeenthaspect of the invention.

In a composite material related to the invention, Au, Ag, Cu and Al withhigh electrical conductivity are used as metals and, particularly, Cu isthe best because of its high melting point and high strength. As aninorganic compound of the composite material, it is undesirable to useconventional compounds with hardness very different from that of a basemetal, such as SiC and Al₂O₃, as mentioned above. It is desirable to usea compound having a granular shape, relatively low hardness and anaverage coefficient of linear expansion in a temperature range from roomtemperature to 300° C. of not more than 10×10⁻⁶/° C. and, morepreferably, not more than 7×10⁻⁶/° C. Copper oxide, tin oxide, leadoxide and nickel oxide are available as such inorganic compounds.Particularly, copper oxide with good ductility is preferred because ofits high plastic workability.

A method of manufacturing a composite material related to the inventioncomprises the steps of melting and casting a raw material comprisingcopper and copper oxide, performing heat treatment at 800° C. to 1050°C., and performing cold or hot plastic working.

Further, a method of manufacturing a composite material related to theinvention comprises the steps of melting and casting a raw materialcomprising copper or copper and copper oxide under a partial pressure ofoxygen of 10⁻² Pa to 10³ Pa, performing heat treatment at 800° C. to1050° C., and performing cold or hot plastic working.

Either cuprous oxide (Cu₂O) or cupric oxide (CuO) may be used as the rawmaterial. The partial pressure of oxygen during melting and casting ispreferably 10⁻² Pa to 10³ Pa and is more preferably 10⁻¹ Pa to 10² Pa.Further, by changing the mixture ratio of the raw material, partialpressure of oxygen, and cooling rate during solidification, etc., it ispossible to control the ratio of the Cu phase to the Cu₂O phase and thesize and shape of the Cu₂O phase of the composite material. Theproportion of the Cu₂O phase is preferably in the range of 10 to 55 vol.%. Especially when the Cu₂O phase becomes more than 55 vol. %, thethermal conductivity decreases and the variation of the properties of acomposite material occurs, making it inappropriate to use the compositematerial in a heat-radiating board of a semiconductor equipment.Regarding the shape of the Cu₂O phase, the shape of a dendrite formedduring solidification is preferred. This is because in the dendritebranches are intricate in a complicated manner, with the result that theexpansion of the Cu phase having large thermal expansion is pinned bythe Cu₂O phase having small thermal expansion. The branches of thedendrite formed during solidification can be controlled, by changing theblending ratio of the raw material or partial pressure of oxygen, tohave a Cu phase, to have a Cu₂O phase, or to have a CuO phase. Also, itis possible to increase strength by dispersing the granular, fine Cu₂Ophase in the Cu phase with the aid of a eutectic reaction. The size andshape of the Cu₂O phase can be controlled by performing heat treatmentat 800° C. to 1050° C. after casting. Furthermore, it is also possibleto transform CuO (which had been formed during solidification) into Cu₂Oby use of internal oxidation process in the above heat treatment. Inother words, this operation is based on the fact that, when CuO coexistswith Cu, the transformation of CuO into Cu₂O by the following formula(1) is thermally more stable at high temperatures:

2Cu+CuO→Cu+Cu₂O  (1)

A predetermined period of time is required in order that Formula (1)reaches equilibrium. For example, when the heat treatment temperature is900° C., about 3 hours are sufficient. The size and shape of the fineCu₂O phase formed in the Cu phase by a eutectic reaction can becontrolled by the heat treatment.

Regarding a method of melting, in addition to ordinary casting, aunidirectional casting process, a thin-sheet continuous casting processand etc. may be used. In the ordinary casting, dendrites areisotropically formed and, therefore, the composite material is madeisotropic. In the unidirectional casting process, the Cu phase and Cu₂Ophase are oriented in one direction and, therefore, anisotropy can beimparted to the composite material. In the thin-sheet continuous castingprocess, dendrites become fine because of a high solidification rateand, therefore, dendrites are oriented in the sheet-thickness direction.For this reason, anisotropy can be imparted to the composite material ofsheet and, at the same time, it is possible to reduce the manufacturingcost.

Further, in a composite material of the invention, since the Cu phaseand Cu₂O phase constituting the composite material are low in hardnessand have sufficient ductility, cold or hot working, such as rolling andforging, is possible and is performed as required after casting or heattreatment. By working the composite material, anisotropy occurs thereinand besides its strength can be increased. Particularly when cold or hotworking is performed, the Cu₂O phase is elongated and oriented in theworking direction and anisotropy in the thermal and mechanicalproperties occurs in the direction at right angles to the elongateddirection. At this time, the thermal conductivity in the elongated andoriented direction is higher than the thermal conductivity at rightangles to the oriented direction, and this difference becomes 5 to 100W/m·k.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical micrograph showing a micro-structure of a samplerelated to Example 1 of the invention.

FIG. 2 is an optical micrograph showing a micro-structure of a samplerelated to Example 2 of the invention.

FIG. 3 is an optical micrograph showing another microstructure of asample related to Example 2 of the invention.

FIG. 4 is an optical micrograph showing a micro-structure of a samplerelated to Example 3 of the invention.

FIG. 5 is an optical micrograph showing a micro-structure of a samplerelated to Example 4 of the invention.

FIG. 6 is a plan view of an IGBT module related to Example 5 of theinvention.

FIG. 7 is a sectional view of an IGBT module related to Example 5 of theinvention.

FIGS. 8A to 8D are schematic drawings showing a manufacturing process ofan IGBT module related to Example 5 of the invention.

FIG. 9 is a graph showing the amount of warp of the base in each step ofmanufacturing process of an IGBT module related to Example 5 of theinvention.

FIGS. 10A, 10B and 10C are a plan view, a sectional view and anequivalent circuit view of a power conversion device in which an IGBTmodule related to Example 5 of the invention is mounted, respectively.

FIG. 11 is a graph showing the amount of warp before the mounting of apower conversion equipment in which an IGBT module related to Example 5of the invention is mounted.

FIG. 12 is a graph showing the amount of warp after the mounting of apower conversion equipment in which an IGBT module related to Example 5of the invention is mounted.

FIG. 13 is a sectional view of a plastic package with a built-inheat-radiating board related to Example 6 of the invention.

FIG. 14 is a sectional view of a plastic package with an exposedheat-radiating board related to Example 6 of the invention.

FIG. 15 is a sectional view of a ceramic package related to Example 7 ofthe invention.

FIG. 16 is a sectional view of a ceramic package provided with aheat-radiating fin related to Example 7 of the invention.

FIG. 17 is a sectional view of a semiconductor equipment related toExample 8 of the invention.

FIG. 18 is a sectional view of a semiconductor equipment related toExample 8 of the invention.

FIG. 19 is a sectional view of an MCM related to Example 9 of theinvention.

FIG. 20 is a sectional view of an electrostatic adsorption devicerelated to Example 10 of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

TABLE 1 Composition Coefficient of Thermal (Vol.%) linear expansionconductivity No. Cu Cu₂O (10⁻⁶/° C.) (W/m · k) 1 90 10 16.0 342 2 80 2014.5 298 3 70 30 13.1 253 4 60 40 11.0 221 5 50 50 10.1 175

Composite materials were produced by casting a raw material obtained bymixing copper and Cu₂O with a purity of 2N at the ratios shown in Table1 after melting it under atmospheric pressure. The coefficient of linearexpansion, thermal conductivity and hardness of these compositematerials were measured. The coefficient of linear expansion wasmeasured in the temperature range from room temperature to 300° C.through the use of a standard sample of SiO₂ with the aid of a push rodtype measuring device. Thermal conductivity was measured by the laserflash method. The results of these measurements are shown in Table 1.The microstructure (100×) of the obtained sample No. 3 is shown in FIG.1. The field of view is 720×950 μm. As shown in the figure, copper oxideis formed to have dendritic shapes and besides granular grains areobserved mostly with grain sizes of 10 to 50 μm with the exception ofone lumpy grain with a diameter of 100 μm. Further, there are bar-likeones of not more than 30 μm in diameter and not less than 50 μm inlength and dendritic ones. The number of these bars and dendrites isabout 10. In addition, the matrix contains granular grains each having agrain size not more than 0.2 μm each of which is spaced about 0.5 μmfrom each dendrite, that is, there are non-formation zones of 0.5 μm inwidth between the granular one and the dendritic one. Further, there arealso granular ones of not more than 0.2 μm in grain size which lie inthread-like line.

As is apparent from Table 1, the coefficients of thermal expansion andthermal conductivity vary in wide ranges by the adjustment of theproportions of Cu and Cu₂O, and it has become evident that thecoefficient of thermal expansion and thermal conductivity can becontrolled to have thermal characteristics required in a heat-radiatingboard.

On the other hand, as is apparent from the microstructure shown in FIG.1, Cu₂O becomes dendrites and the composite material has a finestructure in which the Cu phase and Cu₂O phase are substantiallyuniformly dispersed. Incidentally, the white and black portions in thephotograph represent the Cu phase and Cu₂O phase, respectively.

The results of the hardness measurement reveal that the hardness of theCu phase is Hv 75 to 80 and that the hardness of the Cu₂O phase is Hv210 to 230. As a result of an evaluation of machinability by lathing anddrilling, it has become evident that the machinability is so excellentthat it is easy to obtain any intended shape from the compositematerial.

EXAMPLE 2

TABLE 2 Thermal conductivity (W/m · k) Coefficient of linearLongitudinal Composition expansion (10⁻⁶/° C.) direction (Vol. %)Longitudinal Transverse Longitudinal Transverse Transverse No. Cu Cu₂Odirection direction direction direction direction 7 80 20 14.3 14.7 320275 1.16 8 60 40 11.0 10.9 231 208 1.11

Composite materials were produced by the unidirectional solidificationprocess by casting a raw material obtained by mixing copper and Cu₂Owith a purity of 3N at the ratios shown in Table 2 after melting itunder different partial pressures of oxygen. The microstructure (100×)of the sample No. 7, which was cast after melting in an atmosphere of anoxygen partial pressure of 10⁻² Pa, is shown in FIG. 2. As is apparentfrom the photograph, some of the Cu₂O phase become dendrites and besidesgranular 10 grains are observed mostly with grain sizes of 5 to 50 μm.Further, in the structure, there are linearly lying bar-like ones anddendritic ones each of which bar-like and dendritic ones is not morethan 30 μm in diameter and not less than 50 μm in length. The number ofthese ones is about 16. One lumpy grain with a diameter of not less than100 μm can be seen. In the matrix, most of the Cu₂O phase are granularones not more than 0.2 μm in grain size and thread-like ones lying toform network. Regarding the fine Cu₂O grains in the matrix, it is notedthat there are nonformation zones similarly to the case of FIG. 1.

The microstructure (100×) of the sample No. 8, which was obtained bycasting after melting in an atmosphere of an oxygen partial pressure of10³ Pa, is shown in FIG. 3. As is apparent from the photograph, the Cu₂Ophase forms dendrites and the structure is oriented in one direction. Ithas become also apparent that the shape and density of the Cu₂O phasecan be controlled by changing the raw material and partial pressure ofoxygen. As shown in the figure, there are granular ones with a grainsize of 5 to 30 μm, dendritic ones and bar-like ones of not more than 30μm in diameter and not more than 50 μm in length. The number of thesedendritic ones and bar-like ones is about 33, and the longest one has alength of about 200 μm. Similarly to the cases of the compositematerials shown in FIGS. 1 and 2, the matrix contains granular grainseach having a grain size not more than 0.2 μm, and there arenonformation zones between the granular grain, bar-like one, anddendritic one, and in this example these ones are formed densely allover the matrix, so that an area where the fine grains are formedbecomes small.

In Table 2 are shown the measurement results of the coefficient oflinear expansion and thermal conductivity of the above two kinds ofcomposite materials. From the results, it has been observed that in eachof the composite materials, anisotropy occurs regarding the coefficientof linear expansion and thermal conductivity. The longitudinal directionis the solidification direction of castings and the transverse directionis a direction vertical to the solidification direction. The coefficientof linear expansion is slightly larger in the longitudinal directionthan in the transverse direction when the Cu₂O content becomes not lessthan 30 vol. %, and thermal conductivity becomes not less than 1.1 timeslarger in the longitudinal direction than in the transverse direction

Incidentally, even by blowing oxygen gas into the melt of the rawmaterial, the same result as in the case where oxygen was used as theatmosphere gas was obtained.

EXAMPLE 3

TABLE 3 Coefficient of linear Thermal conductivity Composition expansion(10⁻⁶/° C.) (W/m · k) (Vol. %) Working Longitudinal TransverseLongitudinal Transverse No. Cu Cu₂O condition direction directiondirection direction 9 50 40 900° C., 90% 11.3 10.9 242 198

The above sample No. 8 was hot worked at 900° C. up to a working ratioof 90%. As a result, it has become evident that workability is good andthat the composite materials of the invention are excellent in plasticworking. FIG. 4 shows the microstructure (100×) of the sample No. 9shown in Table 3. In comparison with an as-cast composite material,there was obtained a structure in which an orientating property becomesremarkable and in which the Cu₂O phase is elongated in the plasticworking direction to be thereby made longer in one direction, wherebythe structure of this sample comes to have an aspect ratio in the rangeof from 1 to 20. The diameter of bar-like one is not more than 20 μm andmostly 1 to 10 μm. The number of CuO₂ having a bar-like shape of notless than 100 μm in length is about 15. Fine grains of not more than 0.2μm in an as-cast state grew to grains of about 2 to 5 μm. Further, asshown in Table 3, in the sample No. 9, more remarkable anisotropy isobserved regarding the coefficient of linear expansion and thermalconductivity. In particular, the thermal conductivity in thelongitudinal direction along the bar-like ones is 1.22 times larger thanthe thermal conductivity in the transverse direction. The coefficient oflinear expansion is slightly larger in the longitudinal direction thanin the transverse direction.

EXAMPLE 4

TABLE 4 Coefficient of linear Thermal conductivity Composition expansion(10⁻⁶/° C.) (W/m · k) (Vol. %) Working Heat treatment LongitudinalTransverse Longitudinal Transverse No. Cu Cu₂O condition conditiondirection direction direction direction 10 60 40 900° C., 90% 900° C. ×3 hrs. 11.1 10.9 234 210

FIG. 5 shows the microstructure (100×) of the 10 sample No. 10 shown inTable 4, which was obtained by subjecting the above sample No. 9 to heattreatment at 900° C. for 3 hours. By the heat treatment, the Cu₂O phasewas elongated in the plastic working direction, and almost all grainswere coarsened to have bar diameters of 5 to 30 μm while keeping theirorientating property. As a result, the number of bar-like ones of notless than 100 μm in length becomes about 50, and they lies with longerlengths than before the heat treatment. Further, fines grains grew tograins with a grain size of 2 to 5 μm, so that such fine grainsdisappeared. As shown in Table 4, anisotropy in the coefficient oflinear expansion and thermal conductivity of this sample decreased incomparison with the sample No. 9, and thermal conductivity increased ineach direction while compensating for this decrease in anisotropy.Therefore, the anisotropy in the coefficient of linear expansion andthermal conductivity was able to be controlled through the control ofthe structure by working or heat treatment after working. The thermalconductivity in the longitudinal direction was 1.11 times larger thanthe conductivity in the transverse direction.

EXAMPLE 5

In this example, a copper composite material of the invention wasapplied to a heat-radiating board (base board) of an insulated gatebipolar transistor module (hereinafter abbreviated as an IGBT module),which is one of power semiconductor devices.

FIG. 6 is a plan view of the interior of the module and FIG. 7 is asectional view of a part of the module.

The IGBT elements of 1014 pieces and diode elements of 1022 pieces arebonded to an AlN substrate 103 by use of solder 201. This AlN substrate103 is formed by bonding copper foil 202 and 203 to an AlN board 204 byuse of a silver brazing material not shown in the figure. On the AlNsubstrate 103 are formed regions for an emitter 104, a collector 105 anda gate 106. An IGBT element 101 and a diode element 102 are soldered tothe region for the collector 105. Each element is connected to theemitter 104 by means of a metal wire 107. Further, a resistor element108 is disposed in the region for the gate 106, and a gate pad of IGBTelement 101 is connected to the resistor element 108 by means of themetal wire 107. Six AlN substrates 103 on each of which a semiconductordevice is mounted are bonded to a base material 109 comprising a Cu—Cu₂Oalloy relating to the invention by use of solder 205. Between insulatingsubstrates, interconnect is performed by solder 209 which connects aterminal 206 of a case block 208, in which the terminal 206 and a resincase 207 are integrated, to the AlN substrate 103. Further, the case 207and the base material 109 are bonded to each other withsilicone-rubber-based adhesive 210. Regarding the terminal interconnectof the case block 208, main terminals are connected on each AlNsubstrate 103 to two points with respect to each of emitter terminalinterconnect position 110, emitter sense terminal interconnect position111 and collector terminal interconnect position 112, and are connectedto one point with respect to gate terminal interconnect position 113.Next, a silicone gel 212 is poured from a case lid 211 provided with aresin pouring port so that the whole terminal surface is coated, and athermosetting epoxy resin 213 is then poured over the whole surface,thereby completing the module.

TABLE 5 Coefficient of Thermal linear expansion conductivity Material(10⁻⁶/° C.) (W/m · k) Remarks Cu-30 vol % Cu₂O 13 253 The invention Cu17 390 Conventional MO  5 140 structure Al-SiC  8  16

In Table 5 are shown the coefficient of thermal expansion and thermalconductivity of usually used base materials and those of Cu and 30 vol.% Cu₂O, which is one of the Cu—Cu₂O alloy materials of the inventionobtained in Examples 1 to 5. In semiconductor devices in which the basematerial of Cu—Cu₂O is used, the coefficient of thermal expansion issmall in comparison with usually used modules of Cu base and, therefore,the reliability of the solder 209 which bonds the AlN substrate 103 tothe base material 109 can be improved. On the other hand, insemiconductor devices of Mo base or Al—SiC base used to improve thereliability of solder under severe service conditions, the thermalconductivity is also small although the coefficient of thermal expansionis small in comparison with the semiconductor devices in which the basematerial of Cu—Cu₂O is used, with the result that the problem of largethermal resistance of a module arises. In a module in which the base ofCu—Cu₂O of this example is used, it is possible to ensure thatreliability (life in the thermal fatigue test) is not less than 5 timeslarger than that of a module in which a base of Cu is used and thatthermal resistance is not more than 0.8 time less than that of a modulein which a base of Mo is used when the thickness of the bases is thesame.

These effects enable the range of choices in the structure of a moduleand other members to be widened. For instance, in the example shown inFIG. 6, because a base material of Cu—Cu₂O alloy has a larger thermalconductivity than a base material of Mo, in other words, because itprovides improved heat-spreadability, a temperature difference betweenthe ends and the middle of a semiconductor device during operation canbe reduced to a small amount and, therefore, the size of thesemiconductor device can be made about 1.2 times larger than that of aconventional module. This enables the module to be designed with 24 IGBTelements in comparison with the use of 30 IGBT elements in aconventional semiconductor device in order to ensure the same amount ofcurrent, and the module size could be made smaller. Further, it ispossible to use an alumina substrate, which has a thermal conductivityabout 20% smaller than AlN, as the insulating substrate. Alumina has ahigher toughness than AlN, and the substrate size can be made larger.Further, the alumina substrate has a larger coefficient of thermalexpansion than an AlN substrate and the difference in thermal expansionfrom the base material can be made smaller and, therefore, the amount ofwarp of the module itself can also be reduced. Because the use of analumina substrate enables the allowable substrate size to be larger, thenumber of semiconductor devices capable of being loaded on one substratecan be increased. In other words, it is possible to reduce the areanecessary for ensuring insulation for each insulating substrate and toreduce the area between substrates and, therefore, the module size canbe smaller.

FIGS. 8A to 8D are schematic drawings of the manufacturing process of amodule of this example. In FIG. 8A, the base material 109 comprisingCu—Cu₂O is made to have a substantially flat surfaces coated with Ni. InFIG. 8B, the AlN substrate 103 to which the IGBT element 101 (which is asemiconductor device,) is soldered is bonded to the base material 109 byuse of solder 205. At this time, since the coefficient of thermalexpansion of the base material 109 is larger than that of a compositebody 301 which comprises the semiconductor device and the AlN substrate,the back face of the module is warped in concave shape during thecooling of the solder. In FIG. 8C, in the step of assembling the caseblock 208 with a thermosetting adhesive, since the coefficient ofthermal expansion of the case is larger than that of the composite body301 the soldering of which had been completed, the back face of themodule becomes almost flat during the cooling of the adhesive. In FIG.8D, by filling the interior of the module with the silicone gel 212 andthermosetting epoxy resin 213, the back face of the module is warped inconvex shape because the thermal expansion of the resin is large.

FIG. 9 shows the result of the measurement of the amount of the warp ofthe back face in each step. When the Cu—Cu₂O base of the invention isused, the amount of warp can be decreased to about ⅓ that of a module inwhich the conventional base of Mo is used. Further, in the case of abase of Cu, whose result is not shown in the figure, the difference inthe coefficient of expansion from the AlN substrate is large, so thatthe back face of the module warps in concave shape with a large warpamount during the step of FIG. 8B, and the back face becomes concavewith a warp of not less than 100 μm even after the completion of amodule. In the Cu—Cu₂O base of the invention the amount of warp of themodule can be reduced and, therefore, it is possible to make the size ofthe module larger. Moreover, similarly to the amount of warp in theassembling steps, the amount of change in warp due to temperaturedifferences during the operation of the module is also small, so thatthe outflow of the grease applied between the module and the cooling fincan be prevented.

FIG. 10 shows an embodiment of a power conversion device to which amodule of the invention is applied. In this example, a module 501 wasmounted on a heat sink 511 by means of locking bolts 512 with aheat-radiating grease 510 sandwiched between the module and the heatsink, whereby a two-level inverter was formed. In general, the modules501 of a power semiconductor equipment are mounted in a laterallyreverse relation to each other so that the midpoint (Point B) can beinterconnected at one midpoint interconnect 503. The u-, v- and w-phasesare connected to each of the collector-side interconnect 502 and theemitter-side interconnect 504, and power is supplied thereto from asource 509. A signal conductor is formed of a gate interconnect 505, anemitter auxiliary interconnect 506 and a collector auxiliaryinterconnect 507 of the module 501 of each IGBT. Numeral 508 denotes aload.

FIGS. 11A and 11B and FIGS. 12A and 12B show the amounts of warp of theback face of the module (grease thickness) measured, respectively,before and after the tightening of the module when the module wasmounted. In these figures, FIGS. 11A and 12A show the module in whichthe Cu—Cu₂O alloys of the invention shown in Examples 1 to 4 are used,and FIGS. 11A and 12B show the module of the conventional method. In thecase of the conventionally known Al—SiC base module, the amount ofconvex warp of the back face is about 100 μm. However, when the moduleis tightened while applying grease, the module is deformed duringtightening by being pressed by the grease, so that the back face of themodule is inversely deformed in concave shape with the grease thicknesslarger in the middle thereof, resulting in an increase in contactresistance. In contrast to this, in the case of the Cu-30 vol. % Cu₂Obase of the invention, the amount of initial warp of back face is about50 μm. However, because of large rigidity of the base material, thegrease thickness in the middle of the module after grease applicationand tightening was suppressed to be about 50 μm, i.e., to be half thegrease thickness in the conventional Al—SiC base. Further, it is alsopossible to reduce variations in the grease thickness within the module.The problem of deformation of a module which occurs during mountingbecause the module is forced by grease naturally arises even when a Cubase module of smaller rigidity than the module of Cu—Cu₂O alloy base ismounted. This problem can be solved by using the module of Cu—Cu₂O alloybase of the invention.

As shown in the figures, the Cu—Cu₂ alloy base of the invention canprovide smaller thermal resistance and smaller contact thermalresistance than such base materials as Mo and Al—SiC applied toconventional high-reliability modules. As a result, the module was ableto be mounted in a closely packed condition as shown in FIG. 10.Moreover, because the cooling efficiency of a cooling fin can bedecreased, the mounting area and volume of a power conversion device canbe reduced. Also, because grease thickness can be reduced, the allowablerange of flatness of a cooling fin can be set wide and, therefore, it ispossible to assemble a power conversion device by use of a large fin.Furthermore, the auxiliary cooling function such as forced cooling, etc.can be eliminated, and in this respect, small size design and low noisedesign can be also adopted.

EXAMPLE 6

A heat-radiating board made of each of the composite materialscomprising a copper-copper oxide alloy of the invention described inExamples 1 to 4 was applied to the plastic packages in each of which anIC shown in FIG. 13 and 14 was mounted. FIG. 13 shows a plastic packagewith a built-in heat-radiating board, and FIG. 14 shows a plasticpackage with an exposed heat-radiating board.

The heat-radiating boards were fabricated by changing their chemicalcompositions in the range of Cu-20 to 55 wt.% Cu₂O so that thecoefficient of thermal expansion in a temperature range from roomtemperature to 300° C. becomes 9×10⁻⁶/° C. to 14×10⁻⁶/° C., while takingthe coefficient of thermal expansion of molding resin intoconsideration, and they were used after machining and Ni platingtreatment.

In referring to FIG. 13, the structure of the package is explainedbelow. A lead frame 31 is bonded to an Ni-plated heat-radiating board 33made of a copper composite material of the invention via an insulatingpolyimide tape 32. An IC 34 is bonded to the heat-radiating board 33 byuse of solder. Further, an Al electrode on the IC is connected to thelead frame by means of an Au wire 35. With the exception of a part ofthe lead frame, they are encapsulated with a molding resin 36 whose maincomponents are epoxy resin, silica filler and curing agent. The packagewith an exposed heat-radiating board shown in FIG. 14 differs from thepackage shown in FIG. 13 in the respect that the heat-radiating board 33is exposed outside the molding resin.

The packages mounted as mentioned above were observed regarding whetheror not warps and cracks in the connections between the heat-radiatingboard and the molding resin are present. As a result, it has been foundthat there is no problem when the difference in thermal expansionbetween the molding resin and the heat-radiating board is not more than0.5×10⁻⁶/° C. and that in terms of chemical composition, Cu-20 to 35 wt.% Cu₂O with a high thermal conductivity of 200 W/m·k is preferred.

EXAMPLE 7

FIGS. 15 and 16 show the cross sections of ceramic packages in which oneof the copper composite materials of the invention described in Examples1 to 4 is used as a heat-radiating board and in which an IC is mounted.First, the package shown in FIG. 15 is explained. An IC 41 is bonded toan Ni-coated heat-radiating board 42 by use of a polyimide-base resin.Further, the heat-radiating board 42 is bonded to a package 43 of Al₂O₃with solder. Cu interconnect is provided in the package, which isprovided with a pin 44 for the connection with an interconnectsubstrate. An Al electrode on the IC and the interconnect on the packageare connected by means of an Al wire 45. In order to encapsulate them, aweld ring 46 of kovar was bonded to the package by use of silver solder,and the weld ring and a lid 47 of kovar were welded together by means ofa roller electrode. FIG. 16 shows a package obtained by bonding aheat-radiating fin 48 to the ceramic package shown in FIG. 15.

EXAMPLE 8

FIGS. 17 and 18 show packages in which TAB (tape automated bonding)technology is applied and each of the copper composite materials of theinvention described in Examples 1 to 4 is used as the heat-radiatingboard.

First, the package shown in FIG. 17 is explained. An IC 51 is bonded toa heat-radiating board 53 related to the invention via a heat-conductingresin 52. Au bumps 54 are formed in terminals of the IC, which areconnected to TAB 55, and the TAB is in turn connected to a lead frame 57via a thin-film interconnect 56. The IC is sealed by a ceramic substrate59 of Al₂O₃, a frame 60 and a sealing glass 61 while interposing asilicone rubber 58.

FIG. 18 shows a resin-encapsulated package. An IC 65 is bonded to anNi-plated heat-radiating board 67 related to the invention by use of anAu—Si alloy 66 and is further bonded to both of a copper grounding board69 and an Ni-plated heat-radiating board 70 of the invention by use of aheat-conducting resin 68. On the other hand, the terminals of the IC arebonded to TAB 72 with Au bumps 71 and are encapsulated by use of a resin73. In this package, a part of a lead frame and heat-radiating board areexposed to the outside of the encapsulating resin. The TAB is fixed tothe copper grounding board by use of an epoxy-base Ag paste 74.

EXAMPLE 9

FIG. 19 shows an embodiment of MCM (multi-chip model) in which each ofthe copper composite materials of the invention described in Examples 1to 4 is used as the heat-radiating board. An IC 81 is connected by Auwire 82 to a thin-film interconnect 84 formed on an Ni-platedheat-radiating board 83 of the invention, the IC being further connectedto an interconnect formed on a package 85 of AlN by the Au wire, and theIC is taken out as an external terminal 86. The IC portion is sealed bya lid 87 of 42-alloy while interposing and bonding a preform 88 of Au—Snalloy between the lid 87 and the W-metallized layer of the package.

EXAMPLE 10

FIG. 20 is a sectional view of an electrostatic adsorption device inwhich a composite material of the invention is used.

As shown in FIG. 20, this electrostatic adsorption device can be used asa chuck for a sputtering device which performs the working of aworkpiece 90 of a conductor or semiconductor under a reduced pressure ina vacuum treatment chamber 95. When a voltage (about 500 V) is appliedto an electrode 94 of this electrostatic adsorption device from a DCpower unit 91, the workpiece 90 can be adsorbed on the surface of adielectric board 92. The dielectric board used in this example wasfabricated from each of the composite materials comprising acopper-copper oxide alloy described in Examples 1 to 4.

In performing actual sputtering, by driving an evacuation pump connectedto a gas exhaust port 97 after the mounting of the workpiece 90 ontothis electrostatic adsorption device, the vacuum treatment chamber 95was evacuated until the internal pressure in the chamber became about1×10⁻³ Pa. After that, by opening a valve attached to a gas inlet 96,about 10 SCCM of reaction gas (argon gas, etc.) was introduced into theinterior of the vacuum treatment chamber 95. The internal pressure inthe vacuum treatment chamber 95 at this time was about 2×10⁻² Pa.

After that, by supplying high-frequency power (13.56 MHz) of about 4 kWfrom an electrode 94 of this electrostatic adsorption device, a plasmawas generated between the electrode 94 of this electrostatic adsorptiondevice and another electrode (not shown in the figure). In this case,the applied high-frequency voltage V_(DC) and V_(PP) were 2 kV and 4 kV,respectively. Incidentally, a matching box 98 disposed between theelectrode 94 of this electrostatic adsorption device and ahigh-frequency power unit 93 was used for ensuring impedance matchingwith the vacuum treatment chamber 95 so that high-frequency power wasefficiently supplied to a plasma.

As a result of the actual use of this sputtering device, although thetemperature of the workpiece 90 reached about 450° C. during working, nocracks, etc. that might cause the occurrence of foreign matter wereobserved in the dielectric board 92 of this electrostatic adsorptiondevice. This means that the use of this electrostatic adsorption deviceis useful for an improvement in the reliability of working.

Incidentally, it is needless to say that the same effect of animprovement in working as observed in the chuck for the sputteringdevice is also achieved when this electrostatic adsorption device isused as a chuck for a working apparatus for applying working to aworkpiece of a conductor or semiconductor (for example, a siliconsubstrate) in an atmosphere under a reduced pressure (which workingapparatus is so-called as a working apparatus under a reduced pressure,and include, for example, a chemical vapor-phase deposition device,physical vapor deposition device, milling device, etching device and ionimplantation device).

According to this example, the heat resistance of a dielectric board ofan electrostatic adsorption device can be improved without a decrease inthe dielectric break-down strength of the dielectric board. Therefore,by using an electrostatic adsorption device of the invention as a chuckfor a device for performing working under a reduced pressure, theoccurrence of foreign matters caused by cracks, etc. in the dielectricboard can be reduced.

The composite materials of the present invention have excellent plasticworkability and comprise a Cu phase

with high thermal conductivity and a Cu₂O phase with low thermalexpansion. Because the coefficient of thermal expansion and thermalconductivity can be controlled by adjusting the contents of the Cu phaseand Cu₂O phase, the composite materials can be used in a wide range ofapplications as a heat-radiating board mounted in a semiconductorequipment, etc.

What is claimed is:
 1. A composite material comprising a metal and aninorganic compound, most of said compound being granular grains with agrain size of from 5 μm to not more than 50 μm and dendrites, 1 to 10%of the whole of said compound being fine granular grains with a graindiameter of not more than 1 μm.
 2. A composite material comprising ametal and an inorganic compound, said material being provided with acoefficient of thermal expansion or thermal conductivity in asolidification direction larger than that in the direction vertical tothe solidification direction.
 3. A composite material comprising copperand 10 to 55 vol. % copper oxide, said material being provided with acoefficient of linear expansion in a temperature range from a roomtemperature to 300° C. of 5×10⁻⁶ to 17×10⁻⁶/° C., a thermal conductivityat room temperature of 100 to 380 W/m·k, and an anisotropy.
 4. Acomposite material comprising copper and copper oxide, said copper oxidebeing provided with a shape of a bar oriented in one direction, saidmaterial having a coefficient of linear expansion in a temperature rangefrom a room temperature to 300° C. of 5×10⁻⁶ to 17×10⁻⁶/° C., a thermalconductivity at room temperature of 100 to 380 W/m·k, and anotherthermal conductivity in the oriented direction higher than that in adirection making right angles to the oriented direction, and thedifference between the two being 5 to 100 W/m·k.